1. Field of the Invention
The present invention relates to afocal telescopes and, more particularly, to a three field of view reimaging long wave infrared telescope with a fixed medium field of view.
2. Description of Related Art
When viewing a distant object through a telescope system, the observer is commonly interested in performing several separate functions. One of these functions may be a coarse search over a large field of view for the purpose of locating or acquiring previously undetected objects. Another function may be the fine examination over a smaller field of view for purposes of identification, tracking, or discrimination of previously located objects. Still a third such function may provide a very wide field of view for purposes of pilotage, navigation, or driving. Combining these three functions in a scanning forward looking infrared (FLIR) system usually requires three different afocal magnification telescopes employed in front of a common scanner, imager, and detector/dewar assembly.
In applications requiring high resolution (e.g. for target identification and tracking), a high magnification afocal telescope is desirable. This is due to the large entrance aperture required to reduce diffraction effects in the infrared bands, where such effects contribute significantly to the optical blur, to a sufficiently low level to allow observation of target details. For applications requiring very wide fields of view (e.g. for pilotage or navigation), a magnification near or less than unity is desirable. Also, it is advantageous and desirable, when the ratio between the high and low magnification is very large to provide a third afocal magnification between the two extremes to bridge the gap and make target acquisition, observation, and hand-off between the high and low magnifications less taxing to either a human observer or an automated sensing algorithm.
Prior refractive telescope systems have been utilized in whole or part. These refractive optical systems generally have one or more of the following disadvantages:
a) they may allow only two of the three magnification/field of view modes described above;
b) they often do not allow a sufficiently high magnification in the narrow field of view mode;
c) they are often of undesirably large size;
d) they are composed of a prohibitively large number of optical lens elements, such that the overall transmission is significantly lowered;
e) there is an incomplete correction of optical aberrations, most notably chromatic aberrations;
f) the multiple fields of view are not co-axial and cannot pass through a common external port or window;
g) they may leave unacceptably high narcissus levels whereby the detector can view itself by means of reflections from lens surfaces.
One such device which has overcome these disadvantages is illustrated in U.S. Pat. No. 5,229,880, entitled "Three Field Of View Refractive Afocal Telescope" to Spencer et al, assigned to the same Assignee as the present invention, the specification and drawings of which are herein expressly incorporated by reference. In this system, a refractive afocal telescope optical system provides an improved aberration correction utilizing a minimum number of optical lenses. The system also provides three coaxial fields of view in the thermal infrared band. The system enables packaging within a relatively small diameter spherical volume which leaves sufficient room for other required system elements. Also, it provides afocal magnification greater than those previously available in the art.
While this system works satisfactorily for its intended purpose, like other prior art systems, it is a fixed speed telescope, that is, all fields of view operate at the same F number. Thus, the narrow field of view entrance pupil is larger than the medium field of view pupil, and a larger diameter objective lens is required. Also, the entrance pupil location shifts from narrow field of view to medium field of view to wide field of view. While these disadvantages are minor, those skilled in the art continue to work to improve performance.